/Engine/Source/Runtime/Core/Public/Containers/Set.h
Add all items from another set to our set (union without creating a new set)
template<typename OtherAllocator> void Append ( const [TSet](API\Runtime\Core\Containers\TSet)< [ElementType](API\Runtime\Core\Containers\TSet\ElementType), KeyFuncs, OtherAllocator > & OtherSet )